Conventional ink-jet printing methods can be broadly classified into two types, a continuous type and an on demand type. A dissolved-type aqueous ink composition prepared by dissolving a dye in water is mainly used as an ink for use with such ink-jet printing methods.
An ink which is prepared by dissolving various water-soluble dyes in water or in a mixture of water and a water-soluble organic solvent, and if necessary adding thereto various auxiliary components is now mainly used as the aqueous ink. The ink-jet printing methods have a great deal of potential due to the following advantageous properties: their printing processes are simple because they are of a direct printing type; no noises are produced upon printing because they are of a non-impact type; a color image can be easily produced; high-speed printing can be attained; the running cost is low because ordinary paper is usable as a recording medium; and a high-resolution image is obtainable because extremely fine droplets of an ink are jetted on a recording medium.
However, the above conventional methods suffer from some problems, although the problems are slightly different from one another depending upon the type of the methods. Among them, the common problems which should be solved are as follows:
(1) a jetted ink tends to be blurred on an image-receiving sheet, so that a high-quality image cannot be obtained;
(2) an ink jetted on an image-receiving sheet cannot dry in a short time, so that tailing of the ink is caused; and
(3) a nozzle or ink-supplying path gets clogged by the ink, resulting in unstable jetting of the ink.
To solve the above-described problems, various proposals have been made; for instance, Japanese Patent Publication No. 55-29546 discloses an ink into which a specific surface active agent is incorporated in order to decrease the surface tension of the ink, thereby improving absorption of the ink in an image-receiving sheet; Japanese Patent Publication No. 56-57862 discloses a method in which a strong basic material is incorporated into an ink to raise its pH and to chemically dissolve a sizing agent and pulping agent, which are water repellents used for ordinary paper, thereby controlling expansion and absorption of dots of the ink; and Japanese Patent Publication No. 58-13675 discloses a method in which polyvinyl pyrrolidone having a molecular weight of 40,000 or more is incorporated into an ink to control expansion and absorption of dots of the ink in an image-receiving sheet. However, according to the view of the inventors of the present invention, it is considered that these proposals can solve only part of the previously-mentioned problems, and cannot be the complete solutions thereof.